How to Fix Cupped Hardwood Flooring

by Denise Brown

The planks that make up wood flooring expand and contract based on the amount of moisture in the room. Whenever there is too much moisture, the wood expands, forcing the planks together until their centers bend downward and form individual valleys or cups. In rooms that are temperature-controlled, you are less likely to see this problem. If spilled water sets on the wood flooring too long, the hardwood cups quickly. The subfloor and the wood must be dry before you begin to repair cupped flooring. Fans and dehumidifiers can help speed the drying process. As the wood dries, some of the cupping may disappear.

1

Check the moisture level of the wood before you begin the project, using a moisture meter. It may take an entire heating season for the wood to relax so most of the cupping disappears. A difference of 1 percent between the top and bottom of the hardwood floor planks indicates they are not finished drying.

2

Remove everything from the room. Pry off shoe molding carefully with a pry bar so you can reinstall it when you finish the repairs. Cover the doorways with two layers of plastic sheeting to help prevent dust from entering the other rooms of the house. Hold the sheeting in place with painter’s tape. Cover the heat ducts as well and seal them with painter’s tape. Vacuum the floor to remove any debris.

3

Attach 36-grit sandpaper to a floor sander, which you can rent at a home improvement store or rental agency. Put on a dust mask. Turn on the sander’s vacuum to catch dust and then turn on the machine. Work the machine with the grain of the wood to remove the top layer of finish as well as to sand down the edges along each plank caused by the cupping. Change the sanding disc as soon as it begins to wear. Go over the entire floor with the floor sander.

4

Sand around the edge of the room and areas that the floor sander misses with a handheld orbital sander with 40-grit sandpaper attached. Tip the sander slightly to remove finish from deeper cups, if necessary.

5

Vacuum the room again to remove the dust the sander’s vacuum missed as well as to pick up any debris from the sandpaper.

6

Attach 50-grit sandpaper to the floor sander and to the orbital sander. Go over the entire surface of the floor again. This pass with the sandpaper should remove any scratches as well as the remainder of the high places caused by the cupping. Change the sandpaper, as necessary, to get a uniform surface. Vacuum the floor again.

7

Switch to 80-grit sandpaper in both machines to sand the floor smooth for refinishing. Vacuum once again.

8

Open a window in the room. Put on a respirator as you work with the floor finish.

9

Pour some urethane finish into a painter’s pan or small tub. Dip a long-handled lamb’s wool applicator into the finish and apply it to the floor, working with the grain. Set the applicator on a piece of dry floor and push it into an area where you have applied the urethane. By maintaining a wet edge, your brush strokes with the applicator won’t be visible. Cover the entire floor with the urethane and let it dry overnight.

10

Scuff the surface of the urethane lightly using a long-handled pole sander loaded with 120-grit sandpaper. Vacuum the floor.

11

Apply a second coat of urethane in a similar manner to the first. Scuff sand this coat with 150-grit sandpaper after the urethane dries overnight. Vacuum the dust away. Wipe the surface of the floor with a tack cloth to remove any dust that remains. Alternatively, spray a paper towel with rubbing alcohol and wipe the floor.

12

Apply a third coat of urethane. Let it dry overnight. Scuff sand it with 180-grit sandpaper and remove the dust.

13

Add the final coat of urethane with a paintbrush. Do not overlap strokes or brush marks may be visible. Allow the urethane to dry for three days before cleaning the floor and reattaching the shoe molding with brad nails.

Things You Will Need

Moisture meter

Pry bar

Plastic sheeting

Painter’s tape

Dust mask

Floor sander

Vacuum

Sandpaper discs, various grits

Orbital sander

Sandpaper sheets, various grits

Respirator

Urethane finish

Painter’s pan or small tub

Long-handled lamb’s wool applicator

Long-handled pole sander

Tack cloth

Paper towels

Rubbing alcohol

Small sprayer

Paintbrush

Hammer

Brad nails

Tip

Cupping can also occur in rooms with dry humidity. Increase the relative humidity in the room with cupped floors to 20 percent to prevent cupping from the air being too dry.

Moisture from below can affect the moisture content of hardwood floors. If the crawlspace doesn’t have a moisture barrier, cover the space with 6-mil black polyethylene sheeting. Tape the pieces of sheeting together and to the walls to create a dry zone between the ground and the floor above.

About the Author

Denise Brown is an education professional who wanted to try something different. Two years and more than 500 articles later, she's enjoying her freelance writing experience for online resources such as Work.com and other online information sites. Brown holds a master's degree in history education from Truman State University.

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